1. Field of the Invention
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure for such bits.
2. Background Information
An earth-boring drill bit is connected to the lower end of a drill string and is rotated by rotating the drill string from the surface, with a downhole motor, or by both. With weight-on-bit (WOB) applied, the rotating drill bit engages the formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit. The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (“ROP”), as well as its durability or ability to maintain an acceptable ROP.
In oil and gas drilling operations, costs are generally proportional to the length of time it takes to drill the borehole to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section-by-section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section-by-section. This process, known as a “trip” of the drill string, requires considerable time, effort and expense. Since drilling costs are typically one the order of thousands of dollars per hour, it is desirable to employ drill bits which will drill faster and longer, and which are usable over a wider range of formation hardnesses.
One common type of earth-boring bit, referred to as a rolling cone or cutter bit, includes one or more rotatable cone cutters, each provided with a plurality of cutting elements. During drilling with WOB applied, the cone cutters roll and slide upon the bottom of the borehole as the bit is rotated, thereby enabling the cutting elements to engage and disintegrate the formation in its path. The borehole is formed as the cutting elements gouge and scrape or chip and crush the formation. The chips of formation are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
Cutting elements provided on the rolling cone cutters are typically one of two types—inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “insert” bits, while those having teeth formed from the cone material are commonly known as “milled tooth bits.” The shape and positioning of the cutting elements (both teeth and inserts) upon the cone cutters greatly impact bit durability and ROP, and thus, are important to the success of a particular bit design.
The inserts in insert bits are typically positioned in circumferential rows on the rolling cone cutters. Specifically, most insert bits include a radially outermost heel row of inserts positioned to cut the borehole sidewall, a gage row of inserts radially adjacent the heel row and positioned to cut the corner of the borehole, and multiple inner rows of inserts radially inward of the gage row and positioned to cut the bottom of the borehole. The inserts in the heel row, gage row, and inner rows can have a variety of different geometries.
Particular cutting elements may be more well suited in particular types of formations. For example, milled teeth may be more effective in softer formations. However, the relative softness of milled teeth as compared to inserts may cause the teeth to erode and wear rapidly when engaging harder formations. Once the cutting structure is damaged (e.g., teeth worn and/or broken), the rate of penetration may be reduced to an unacceptable rate, the drill string must be removed in order to replace the drill bit. Inserts made of relatively hard materials (e.g., material containing a high percentage of tungsten carbide) are usually more effective in harder formations. However, inserts often have smaller cutting surfaces as compared to milled teeth, reducing their effectiveness in softer formations. Further, formations may contain both relatively hard and soft zones, reducing the effectiveness and drilling efficiency of a rolling cone bit having only either inserts or milled teeth.
Accordingly, there remains a need in the art for drill bits that provide a relatively high rate of penetration and footage drilled, yet are durable enough to withstand hard and abrasive formations that may quickly damage milled teeth of a rolling cone bit. Such drill bits and cutting elements would be particularly well received if they offered the potential to improve overall drilling efficiency in formations including both soft and hard zones without the need for tripping the bit out of the hole in order to exchange drill bits.